FIGS. 1 and 2 illustrate a small airblast fuel nozzle used in the past in small gas turbine engines of, for example, 1000-2000 horsepower. These airblast fuel nozzles include a first nozzle body 10 having a flange 10a by which the nozzle is mounted to the combustor wall 12 and an upstream threaded inlet fitting 14 connected to a fuel conduit. A second nozzle body 16 is attached by welding and the like on the front tubular extension 15 of the first nozzle body. Attached on the downstream end of the second nozzle body is nozzle tip 17 having outer air shroud 18 therearound.
Fuel enters the nozzle through fitting 14 and passes through filter 20 past flow restrictor orifice 22 into chamber 24. Fuel from chamber 24 flows through drilled circumferentially spaced passages 26 to annular chamber 28 and through a transverse slot (not shown) to discharge orifice 30 in the air swirl chamber 32 for atomization by swirling air exiting therefrom.
Swirl air for chamber 32 enters circumferentially spaced air inlet passages 34 which as shown extend in radially and forwardly inclined directions relative to axis A. The air inlet passages intersect the swirl chamber 32 in a tangential manner as shown in FIG. 2. The purpose of air inlet passages 34 is to impart sufficient swirl to air as it enters chamber 32 and flows therealong to effect sufficient atomization of fuel at discharge orifice 30 to ignite same in the presence of a spark ignition.
Outer air passing inside shroud 18 is also swirled by swirl vanes 36 to aid fuel atomization as the previously atomized fuel exits from discharge lip 40 for injection into the combustor.
In these small gas turbine engines low stagnation air pressure; e.g., 1-11/2 inches of water, is available from the compressor section of the engine at cold ignition for entering air inlet passage 34 and swirling along swirl chamber 32. At these low air pressure values, there has been a problem with achieving cold ignition on a consistent basis; i.e., the engines have been difficult to start.
What is needed is a solution to the problem of inconsistent and difficult cold ignition of such small gas turbines having only low stagnation air pressure available from the compressor at cold ignition.